A summary field with the "total of" option might compute the total of the above calculation. That will give you the count of all records in the current found set where the call back is overdue. There are also ways you might use with a relationship or ExecuteSQL to produce the same type of aggregate value.

And you might use the TimeStamp function to take a date and time field as parameters to return the equivalent time stamp as a slightly simpler way to combine the date and time into such a value instead of getastimestamp.